One of the clear implications of psychobiological research is the importance of early events in shaping subsequent development. Observation and manipulation of rodent fetuses has permitted investigation of the process of prenatal behavioral development, and has traced the developmental origins of learning and memory to the prenatal period. The fetal rat model presents advantages over neonatal study, because the fetus lacks experience with stimuli associated with suckling or maternal behavior. The fetus also can be maintained in an experimental setting that does not require active behavioral care by the mother. The rat fetus provides a simple mammalian system for experimental investigation of how associations are acquired and memories are stored within an incomplete nervous system that undergoes constant and rapid change during early ontogeny. Prenatal sensory experience is controlled by presentation of stimuli to individual fetal subjects against a relatively stable sensory background. This proposal introduces tactile and chemosensory stimuli that mimic important features of the neonatal environment, including an artificial nipple and milk. These stimuli reliably evoke behavioral and physiological responses from naive fetal subjects, including activation of the fetal opioid system by milk, and can support fetal learning through their association. Opioid activity also can be manipulated by administration of selective agonist and antagonist drugs to individual fetal subjects. Responses of the fetus are measured by observing motor behavior, including coordinated action patterns. Species-typical motor responses will serve as behavioral bioassays for measuring opioid involvement in fetal associative learning and memory. This program of research will investigate (a) the acquisition of conditioning, (b) the specificity of fetal responses to the artificial nipple, (c) effects of conditioning on opioid responses evoked by milk, (d) memory and retention of experiences with the nipple, and (e) ontogenetic changes in opioid involvement in fetal learning. The use of ecologically relevant and developmentally appropriate sensory manipulations to study fetal learning provides a means for quantitative assessment of the integrated output of the developing nervous system in the intact animal.